1. Field of the Invention
The present invention relates to a lithographic printing plate precursor and a lithographic printing method for using the same. More particularly, the invention relates to a lithographic printing plate precursor of the so-called direct platemaking type, from which a printing plate can be directly obtained through scanning with an infrared laser based on digital signals from, e.g., a computer. The invention further relates to a lithographic printing method in which the lithographic printing plate precursor is developed on a printing machine and used to conduct printing.
2. Description of the Related Art
A lithographic printing plate generally has ink-receptivity image areas, which receive an ink during printing, and hydrophilic non-image areas, which receive a fountain solution. Lithography is a process in which the surface of a lithographic printing plate is made to have a difference in ink adhesion by forming ink-receptivity image areas as ink-receiving areas and hydrophilic non-image areas as fountain-solution-receiving areas (non-ink-receiving areas) based on the fact that water has the property of repelling oil-based inks, and an ink is adhered only to the image areas and then transferred to a material to be printed, e.g., paper, to conduct printing.
A lithographic printing plate precursor (PS plate) comprising a hydrophilic support and an ink-receptivity photosensitive resin layer (image-recording layer) formed thereon has hitherto been in wide use for producing such lithographic printing plate therefrom. Usually, a lithographic printing plate is produced from a lithographic printing plate precursor by a method which comprises exposing the precursor through an original, e.g., a lith film, and then dissolving and removing the image-recording layer in the non-image areas with an alkaline developing solution or organic solvent to thereby expose the corresponding surface of the hydrophilic support while leaving the image-recording layer in the image areas.
Such platemaking processes heretofore in use for producing a printing plate from a lithographic printing plate precursor necessitate a step in which the non-image areas after exposure are dissolved and removed with a developing solution or the like suitable for the image-recording layer. However, to eliminate or simplify such a wet treatment performed additionally is one of the subjects to be accomplished. In particular, the discard of waste liquids resulting from wet treatments has recently become a matter of considerable concern of the whole industrial world from the standpoint of care of the global environment and, hence, there is an increasingly growing desire for the accomplishment of that subject.
For this purpose, a technique called on-press development has been proposed as a simple platemaking method. This technique uses a lithographic printing plate precursor having an image-recording layer whose non-image areas can be removed in an ordinary printing process. After exposure, the non-image areas are removed on a printing machine to obtain a lithographic printing plate.
Examples of the on-press development include: a method which uses a lithographic printing plate precursor having an image-recording layer capable of being dissolved or dispersed in a fountain solution or ink solvent or in a fountain solution/ink emulsion; a method in which an image-recording layer is mechanically removed by contact with rollers or the blanket cylinder of a pressing machine; and a method in which the cohesive force of an image-recording layer or adhesion between the image-recording layer and the support is reduced by the penetration of a fountain solution, ink solvent, or the like and, thereafter, the image-recording layer is mechanically removed by contact with rollers or the blanket cylinder.
On the other hand, digitization technology in which image information is electronically processed, accumulated, and outputted by a computer has recently come to spread extensively, and various new image output techniques suitable for such digitization technology have come to be practically used. Under these circumstances, attention is focused on a computer-to-plate technique in which a highly convergent radiation such as a laser light is caused to carry digitized image information and this light is used to scan and expose a lithographic printing plate precursor to directly produce a lithographic printing plate without via a lith film. Consequently, to obtain a lithographic printing plate precursor suitable for such a technique has become one of important technical subjects.
As described above, simplification of platemaking and use of a dry platemaking process and no development step have recently come to be more strongly desired than before from the standpoints of care of the global environment and suitability for digitization.
However, in the case where the conventional image-recording method, which utilizes a light having wavelengths from the ultraviolet to visible region, is used for the simplification of a platemaking operation, such as on-press development, the image-recording layer remains unfixed after exposure and hence retains sensitivity to indoor light. It has therefore been necessary that the lithographic printing plate precursor taken out of a package should be kept in a completely light-shielded state until on-press development is completed.
High-output lasers such as a semiconductor laser emitting infrared rays having a wavelength of from 760 to 1,200 nm and a YAG laser have recently become available at low cost. Because of this, a easy process for lithographic printing plate production to be incorporated into digitations technology, using any of these high-output lasers as a light source for image recording through scanning exposure is coming to be regarded as a promising process.
In the conventional platemaking process using a light having wavelengths from the ultraviolet to visible region, a photosensitive lithographic printing plate precursor is imagewise exposed at a low to medium illuminance to record an image based on an imagewise property change caused by a photochemical reaction in the image-recording layer. In contrast, in the above-described process using a high output laser, a large quantity of light energy is applied to exposed areas in an extremely short time period to efficiently convert the light energy to heat energy and the image-recording layer is caused by this heat to thermally undergo a change such as a chemical change, phase change, or change in form or structure. This change is utilized for image recording. Consequently, although image information is inputted by means of light energy such as laser light, image recording is influenced not only by the light energy but also by the reaction caused by heat energy. Usually, the recording technique utilizing the heat generated by such high-power-density exposure is called heat mode recording, and the conversion of light energy into heat energy is called light/heat conversion.
Great merits of platemaking processes employing heat mode recording are that the image-recording layer is not sensitive to light on an ordinary illuminance level, such as indoor light, and that an operation for fixing the image recorded by high-illuminance exposure is not essential. Namely, there is no possibility that the lithographic printing plate precursor for use in heat mode recording might be influenced by indoor light before exposure, and it is not essential to conduct an operation for image fixing after exposure. Consequently, when a platemaking process, in which an image-recording layer which is insolubilized or solubilized by exposure using, e.g., a high-output laser and the exposed image-recording layer is made to bear an imagewise to thereby produce a lithographic printing plate, is conducted during on-press development, then a printing system is expected to be possible in which the image is not influenced even when the image-recording layer after the exposure is exposed to indoor ambient light. This system is desired to be realized.
Known as such a lithographic printing plate precursor is, for example, a lithographic printing plate precursor comprising a hydrophilic support and, formed thereon, an image-forming layer comprising a hydrophilic binder and hydrophobic thermoplastic polymer particles dispersed therein (see, for example, Japanese Patent No. 2938397). This lithographic printing plate precursor can be used in the following manner. The precursor is exposed with an infrared laser to thermally fusion-bond the hydrophobic thermoplastic polymer particles to one another and thereby form an image. Thereafter, this precursor is attached to the cylinder of a printing machine, and a fountain solution and/or an ink is supplied thereto to develop the image-forming layer by on-press development.
However, the technique described above in which an image is formed by the mere bonding of fine polymer particles by thermal fusion has been disadvantageous in that image strength is considerably low and printing durability is insufficient, although the lithographic printing plate precursor shows satisfactory on-press developability.
A technique for improving the printing durability of such a lithographic printing plate precursor capable of on-press development has been proposed. It is a lithographic printing plate precursor characterized in that it comprises a hydrophilic support and, formed thereover, a heat-sensitive layer containing microcapsules containing a compound having a functional group reacting by the action of heat, and that an infrared absorber is contained in either the heat-sensitive layer or a layer adjacent thereto (see JP-A-2001-277740 and JP-A-2001-277742).
Another technique for improving printing durability is known. It is a lithographic printing plate precursor capable of on-press development which comprises a support and formed thereon a photosensitive layer comprising an infrared absorber, a radical polymerization initiator, and a polymerizable compound (see JP-A-2002-287334).
Those techniques utilizing a reaction such as polymerization reaction can attain an improvement in image strength because the image areas have a higher chemical-bond density than the image areas formed by the thermal fusion bonding of fine polymer particles. However, those techniques have been still insufficient from the standpoint of satisfying both of on-press developability and thin-line reproducibility or printing durability.